[BACKPORT 2.18][#14308] DocDB: update flag description now that savep…

…oints work with XCluster

Summary:
Original commit: b924861 / D27199

Clean merge to origin/2.18.

original summary

See title.
Also changed this flag stability level from evolving to stable.
Jira: DB-3745

Test Plan:
```
$ yb-master --help|grep -A 1 savepoint
    -enable_pg_savepoints (Set to false to disable savepoints in YugaByte
      PostgreSQL API.) type: bool default: true
```

Reviewers: xCluster, hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D27213

src/yb/common/common_flags.cc

@@ -32,9 +32,8 @@ TAG_FLAG(ysql_disable_index_backfill, advanced);
 
 DEFINE_NON_RUNTIME_bool(
     enable_pg_savepoints, true,
-    "Set to false to disable savepoints in YugaByte PostgreSQL API. "
-    "This needs to be set to false when using xcluster replication for now.");
-TAG_FLAG(enable_pg_savepoints, evolving);
+    "Set to false to disable savepoints in YugaByte PostgreSQL API.");
+TAG_FLAG(enable_pg_savepoints, stable);
 TAG_FLAG(enable_pg_savepoints, advanced);
 
 DEFINE_RUNTIME_AUTO_bool(enable_automatic_tablet_splitting, kNewInstallsOnly, false, true,